This invention generally relates to devices and methods for cleaning the inkjet nozzles of an inkjet printhead, and is specifically concerned with a fluid jet device that cleans such inkjet nozzles by means of a stream of small, high velocity fluid droplets discharged from a cleaning head mounted in opposition to the printhead.
Devices for cleaning the nozzles of either drop-on-demand or continuous inkjet nozzles in a printhead are known in the prior art. Such devices are necessary, as dried ink deposits and other debris tend to accumulate around the orifices of the inkjet nozzles over time. Such deposits and debris may ultimately interfere with the ability of the printhead to achieve high resolution printing by either deflecting the intended trajectory of the ink droplets discharged from the nozzles, or, in extreme cases, blocking the orifices of the nozzles altogether.
In many of these prior art devices, a printhead wiper cleans the nozzle surfaces via a mechanical wiping action. Cleaning solvent is applied either to the wiper itself, or onto the surface of the printhead prior to the wiping operation. In another type of prior art cleaning device, cleaning solution is either oscillated or discharged directly through the nozzles of the inkjet printhead itself. In some prior art devices, the printing ink itself is used as a cleaning solvent prior to the initiation of a printing operation to simplify the cleaning operation.
While mechanical wiping techniques are effective in the removal of contaminants, they also reduce the lifetime of the printhead due to mechanical wear. They are further time consuming and consequently reduce printer productivity. Even in prior art devices where the cleaning fluid is applied without direct mechanical contact to the printhead (as, for example, via a spray nozzle), such application alone is not effective in dislodging and removing the deposits and debris around the inkjet nozzles, and the use of a mechanical wiper is necessary to complete the cleaning operation. Moreover, in all such prior art cleaning devices, no attempt is made to restrict the application of the cleaning fluid to the bore of the nozzles. Consequently, the entire printhead surface (and possibly other portions of the printer) are completely covered with a cleaning solution, which is not only unnecessary and wasteful, but potentially damaging to fragile and sensitive mechanical and electrical components on and around the printhead.
In prior art cleaning devices where the ink itself is the cleaning fluid and is either oscillated within the nozzle or ejected from it, optimal cleaning is not easily achieved due to the fact that neither an oscillatory or a continuously streaming fluid provides much dislodgment force on the contaminants and debris on the interior surfaces of the nozzle. In cases where a cleaning fluid other than ink is used, the cleaning fluid must be completely purged from the printhead and the printhead must be refilled with ink after the cleaning operation. In all cases where the cleaning fluid is ejected from the inkjet nozzles themselves, a large volume of cleaning fluid (whether ink or a special cleaning solution) is necessary.
Clearly, there is a need for a cleaning technique that avoids the mechanical wear associated with wiping techniques, and the waste and ineffectiveness associated with techniques which oscillate or eject cleaning fluids through the inkjet nozzles themselves. Ideally, such a technique would concentrate the cleaning action on or around the inkjet nozzles themselves in order to conserve cleaning fluid, and to eliminate contact between the cleaning fluid and fragile electronic and mechanical components located near the vicinity of the inkjet nozzles. Finally, such a technique should be adaptable to both drop-on-demand and continuous inkjet printers, and rapid in operation in order to minimize printing downtime.
Generally speaking, the invention is a fluid jet apparatus and method that cleans the inkjet nozzles of a printhead without the aforementioned shortcomings associated with the prior art. To this end, the fluid jet apparatus of the invention comprises a cleaning head having an array of cleaning nozzles registrable with the array of inkjet nozzles in the printhead, a mounting assembly that mounts the cleaning head in opposition to the printhead with the cleaning nozzles in substantial alignment with the inkjet nozzles, and a supply of pressurized fluid connected to the cleaning nozzles such that the cleaning nozzles discharge a stream of fluid droplets that impinge on the inkjet nozzles, wherein at least some of the droplets are about the same size as the orifices of the printhead nozzles. The fluid jet apparatus preferably includes a droplet sizing mechanism that controls the size of the cleaning droplets discharged by the cleaning nozzles. Such a droplet sizing mechanism may have a plurality of electrical resistance heaters adjacent to each of the cleaning nozzles for applying heat pulses at different frequencies to the stream of fluid discharged thereby to thermally xe2x80x9cpinchxe2x80x9d the stream into droplets of a desired size.
The fluid jet apparatus may further have a droplet speed controller that controls the velocity and frequency of solvent droplets discharged by the cleaning nozzle. The supply of pressurized fluid may include a fluid pump and the droplet speed controller may include a circuit for controlling the amount of pressure that the pump generates in fluid connected to the cleaning nozzles.
The fluid jet apparatus may also comprise a mechanism for changing a location of impingement of the cleaning droplets with respect to the inkjet nozzles of the printhead. Such a location mechanism may include a cleaning head moving assembly for oscillating the cleaning head relative to the printhead. In another embodiment of the invention, the locating changing mechanism may include a cleaning droplet deflector that deflects a path of cleaning droplets as they are discharged from the cleaning nozzles. Such a deflector may take the form of electrical resistance heaters positioned adjacent to each of the cleaning nozzles for asymmetrically applying heat pulses to the stream of cleaning droplets discharged by the cleaning nozzles. In another embodiment, the location changing mechanism may include a device for generating a fluid stream, such as a stream of air, that traverses the path of the cleaning droplets. The flow rate of the fluid stream varies over time to different areas of the nozzles in order to deflect the cleaning droplets.
The fluid jet apparatus also preferably includes a cleaning fluid reclamation system. Such a system may include a gutter for collecting liquid cleaning fluid that impinges and runs off of the inkjet nozzles of the printhead. The reclamation system may further include a pump for generating a negative pressure in the inkjet nozzles during a cleaning operation such that at least some of the cleaning droplets are sucked into the inkjet nozzles and directed back into a reclamation reservoir.
In operation, the fluid jet apparatus discharges discrete droplets of cleaning fluid of controlled size and high velocity in and around the orifices of the inkjet nozzles. At least some of the droplets are about the same size as the printhead nozzle orifices. The trajectory of the cleaning droplets may be varied during cleaning by oscillating the cleaning head, applying asymmetric thermal pulses or applying a time-varying fluid stream across the droplets so that droplets impinge on different areas of the printhead nozzles. The cleaning droplets may be discharged continuously during the cleaning operation, or on demand by conducting individual slugs of cleaning fluid to the cleaning head between pulses of compressed air to conserve cleaning fluid. In all cases, the resulting high velocity impingement of small individual cleaning droplets provides a highly effective cleaning action with a minimum amount of cleaning fluid that sharply focuses the cleaning action on the nozzle orifices themselves, and in areas immediately surrounding them, thereby preventing other potentially sensitive areas of the printhead from being unnecessarily exposed to the cleaning fluid.